The present invention relates to pilot operated hydraulic valves, and to pilot valves which are pressure balanced.
Construction and agricultural equipment have moveable members which are operated by a hydraulic cylinder and piston arrangement. Application of hydraulic fluid to the cylinder traditionally has been controlled by a manual valve, such as the one described in U.S. Pat. No. 5,579,642. In this type of valve, a manual operator lever was mechanically connected to a spool within the valve. Movement of the spool into various positions with respect to cavities in the valve body enables pressurized hydraulic fluid to flow from a pump to one of the cylinder chambers and be drained from the other chamber. By varying the degree to which the spool was moved, the rate of flow into the associated chamber can be varied, thereby moving the piston at proportionally different speeds.
There is a present trend with respect to construction and agricultural equipment away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located in the operator cab. This change in technology also facilitates computerized control of various machine functions.
Solenoid operated pilot valves are well known for controlling the flow of hydraulic fluid and employ an electromagnetic coil which moves an armature in one direction to open a valve. The armature acts on a pilot poppet that controls the flow of fluid through a pilot passage in a main valve poppet. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or another valve member is spring loaded to close the valve when electric current is removed from the solenoid coil.
A drawback of conventional solenoid operated pilot valves results from effects produced by the pressure differential that develops across the valve in the closed state. That pressure differential changes with variation in the load applied to the equipment component that is operated by the hydraulic fluid from the valve. The load and the supply pressure variation affect the pressure at the valve's outlet and produces that pressure differential across the valve. In the closed state, the pressure differential can affect the amount of force required to open the valve and required to produce a given flow rate of the hydraulic fluid. Therefore in a solenoid operated valve, variation in this pressure differential affects the magnitude of electrical current required to operate the valve.
This problem was addressed by providing a pressure balancing stem in the pilot valve passage, as shown in U.S. Pat. No. 5,878,647. This balanced the pressures so that the only forces acting on the pilot poppet were those from the return spring and the electromagnetic coil. While this overcame the effects due to a pressure imbalance in other designs, fluid leakage often occurred along the balancing stem. Attempts to provide a better seal against that leakage increased friction acting on the stem and created a sticking problem that adversely affected smooth valve operation.